Summary of a Geology research paper entitled “Radiative forcing of climate by ice-age atmospheric dust” by T. Claquin C. Roelandt K.E. Kohfeld S.P. Harrison I. Tegen I.C. Prentice Y. Balkanski G. Bergametti M. Hansson N. Mahowald H. Rodhe M. Schulz
HOW IS THE MEAN RADIATIVE FORCING OF CLIMATE DUE TO DUST QUANTIFIED?
Mineral aerosol(dust) in the atmosphere contributes to radiative forcing of present day climate (Tegen and Fung 1994; Sokolik and Toon 1996).This forcing depends on aerosol optical properties and the albedo of the underlying surface. During glacial periods, dust deposition rates were very high compared to present. Deposition fluxes of dust at last glacial maximum(LGM)were found to be high thus indicating a long term higher global atmospheric concentration of dust than observed at present. (Thompson and Moseley-Thompson 1981; Rea 1994; DeAngelis et al. 1997; Petit et al. 1999),
In the present study, the modelled three dimensional dust fields from Mahowald et al. and modelled seasonally varying surface albedo fields derived in a parallel manner are used to quantify the mean radiative forcing due to modern and LGM dust.
The BIOME3 model(Haxeltine and Prentice 1996)was used to simulate dust source regions. This model is an equilibrium biogeography model which predicts the large scale distribution of major terrestrial ecosystem types(biomes) as a function of climate, soil properties and atmospheric(CO2). In BIOME3, an initial sieve selects the plant functional types that occur in a given environment. Then for each grid cell, the model computes multiple values of annual net primary production(NPP). The modern vegetation distribution was obtained with BIOME3 using an observed modern climatology. The LGM climate scenario was derived from the ECHAM3 atmospheric general circulation model.(Pinot et al. 1999).The offline atmospheric tracer transport model(TM3) was used to simulate global dust transport.(Heimann 1995; Schulz et al. 1998) .
Sea surface albedo measurements were obtained directly from ECHAM3 while land surface albedo was modelled from BIOME3. The DIRTMAP data (Mahowald et al. 1999; Kohfeld and Harrison 2000, 2001) used here suggests that the simulated zonal patterns and their changes between LGM and modern climates are consistent with the zonal patterns of observed recent and LGM dust accumulation rates. These observations support the geographic features seen the simulated dust fields including increased offshore transport from the Sahara(kolla et al.1979; Ruddiman 1997) and Australia(Hesse 1994) during the LGM and an enhancement in the amount of dust transported from central Asia to northern latitudes(Rea and Leinen 1988;Biscaye et al.1997).Dust in the east Antarctic ice cores has been attributed to pantagonian sources(Basile et al.1997) but in the present simulation study, much of the dust deposited at these sites originates from Australia. Among the two major factors that modify the radiative effects of dust, it was found that the simulated particle size distribution did not change substantially between the LGM and modern climates(Steffensen 1997). But the simulated surface albedo values changed substantially and were found to be higher than modern values at high latitudes and in the tropics .
Different optical properties due to different mixing states of minerals like disposition of haematite were considered as a result bounding calculations using alternatives of external and internal mixing of haematite were made. Such experiments showed that the mean LGM dust loading in the high latitudes produced an overall negative radiative effect and this effect in the tropics is mainly due to high absolute concentrations of dust.(Mahowald et al. 1999). Such extensive dust sources in turn help to maintain low sea surface temperatures in these regions by the radiative mechanisms thus maintaining a low CO2.(Martin 1990;Lefevre and Watson 1999; Watson et al. 2000).
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